In recent years, as semiconductor devices are miniaturized, there are needs for miniaturizing a pattern width, a pattern pitch (line width), and so forth, of a circuit pattern formed on a wafer, a resist pattern for forming a circuit pattern, and so forth. These needs can be satisfied by using ultraviolet light having a shorter wavelength to expose the resist pattern. As semiconductor devices are miniaturized, ultraviolet light having short wavelengths is used to expose wafers. For a semiconductor device produced corresponding to a design rule of 350 nm, ultraviolet light having a wavelength of 365 nm is used; for semiconductor devices produced corresponding to design rules of 250 nm and 180 nm, ultraviolet light having a wavelength of 248 nm is used; and for semiconductor devices produced corresponding to design rules of 130 nm and 100 nm, ultraviolet light having a wavelength of 193 nm is used. Currently, ultraviolet light having even a wavelength of 157 nm is used.
It is known that resolutions corresponding to these wavelengths are expressed by Rayleigh's formula, w=k1×(λ/NA), where w represents a minimum width of a pattern that is to be resolved, NA represents a numerical aperture of a lens of a projection optical system; λ represents a wavelength of exposure light; k1 represents a process constant that depends on the performance of the resist pattern, the use of resolution enhancement technology, and so forth. It is known that when the optimum resist pattern and super high resolution technology are used a process constant of k1=around 0.35 can be selected. In the super high resolution technology, ±first order diffraction light that passes through a mask and is diffracted by a light insulating pattern on the mask is selectively used so as to obtain a pattern smaller than a wavelength. Theoretically, a much smaller pattern can be obtained using ±n-th order diffraction light (where n≧2). However, the intensity of the diffraction light remarkably decreases. In addition, the pattern is restricted by the pupil of the projection optical system. Thus, it is not practical to use ±n-th order diffraction light (where n≧2).
According to the Rayleigh's formula, it is found that when ultraviolet light having a wavelength of 157 nm is used and a lens having NA=0.9 is used, the minimum pattern width is 61 nm. In other words, to obtain a pattern width smaller than 61 nm, it is necessary to use ultraviolet light having a shorter wavelength than 157 nm.
Thus, recently, the use of ultraviolet light having a wavelength of 13.5 nm, referred to as extreme ultraviolet light, has been considered for use as ultraviolet light having a shorter wavelength than 157=n. However, there are materials such as calcium fluoride (CaF2) and silicon dioxide (SiO2) that have light transmissivities for ultraviolet light having a wavelength of up to 157 nm. Thus, a mask and an optical system that are capable of transmitting these ultraviolet lights can be produced. However, there are no materials that are capable of transmitting extreme ultraviolet light having a wavelength of 13.5 nm with a desired thickness. Thus, when extreme ultraviolet light having a wavelength of 13.5 nm is used, it is necessary to structure a mask and an optical system as a reflective type mask and a reflection type optical system that reflects light rather than a light transparent mask and a light transmission type optical system, respectively.
When a light reflective type mask and a light reflection type optical system are used, light reflected on a mask surface has to be guided to a projection optical system without interference with light that enters the mask Thus, light that enters the mask should inevitably have an incident angle of φ against the normal line of the mask surface. This angle depends on the numerical aperture NA of the lens of the projection optical system, magnification m of the mask, and the size σ of the lighting source. More specifically, when a mask having a reduction factor of, for example, 5× is used on a wafer, in an exposure apparatus having NA=0.3 and σ=0.8, light enters the mask at a solid angle of 3.44±2.75 degrees. When a mask having a reduction factor of 4× is used on a wafer, in an exposure apparatus having NA=0.25 and σ=0.7, light enters the mask at a solid angle of 3.58±2.51 degrees.
As a reflective type mask that deals with inclined incident light, a mask includes a stack of reflective layers, hereinafter referred to as mask blanks, that reflect extreme ultraviolet light, an absorber film that covers the mask blanks with a predetermined pattern and absorbs extreme ultraviolet light, and a buffer film interposed between the mask blanks and the absorber film The mask blanks are constituted by a structure composed of molybdenum (Mo) layers and silicon (Si) layers that are alternately laminated. Generally, the mask blanks are composed of a total of 40 layers of molybdenum layers and silicon layers. When the absorber film that absorbs extreme ultraviolet light is coated in a predetermined pattern on the mask blanks, incident light is selectively reflected corresponding to a circuit pattern, a resist pattern, and so forth that are to be formed. The buffer film is disposed as an etching stopper that prevents the stack of reflective layers from being etched or to avoid being damaged when defects have been removed from the formed absorber film.
When the foregoing reflective type mask is produced, it is necessary to properly decide the film thickness of the absorber film. Conventionally, the film thickness of the absorber film is decided in accordance with an optical density (hereinafter referred to as “OD”) of the absorber film, for example, OD=3. The film thickness of the absorber film that satisfies OD=3 is a film thickness that causes the intensity of incident light to be attenuated to 1/1000. As described in, for example, “Proceedings of SPIE vol. 4343 (2001) pp 409-414 “TaN EUVL Mask Fabrication and Characterization”, the value of the OD depends on the reflectance of the surface of the absorber film. The film thickness is decided in accordance with the OD obtained from the reflectance of the surface of the absorber film because a transparent mask that is not capable of dealing with extreme ultraviolet light employs that method.
However, when a reflective type mask dealing with extreme ultraviolet light is used, if the film thickness of the absorber film is decided in accordance with the OD obtained from the reflectance of the surface of the absorber film, the line width variation and the pattern shift to be transferred to a wafer may become large. In the case of the reflective type mask, although the absorber film and the buffer film are formed on the mask blanks that reflect extreme ultraviolet light, the swing effect and the bulk effect may occur on a transferred image on a wafer due to multiple reflections in the absorber film and the buffer film.
More specifically, when the thicknesses of the absorber film and the buffer film are approximately 100 nm and 30 nm, respectively, these thicknesses are larger than a wavelength of 13.5 nm of extreme ultraviolet light, the standing wave effect occurs between extreme ultraviolet light and light reflected by the mask blanks. This standing wave periodically varies the line width and position of the pattern to be transferred to the wafer. In other words, the swing effect, in which the line width and pattern position periodically swing, and the bulk effect, in which they do not periodically vary, occur.
Thus, when the film thickness of the absorber film is decided in accordance with the OD obtained from the reflectance of the surface of the absorber film, the film thickness of the absorber film may be decreased so as to reduce the influence of the light insulating effect of the absorber film (for example, OD=2). However, the foregoing swing effect and bulk effect may cause the line width variation and pattern shift to become too large. As a result, the needs for miniaturizing the pattern width, pattern pitch, and so forth of a transferred image would not be properly satisfied.
Therefore, an object of the present invention is to provide an exposure mask, a method for producing an exposure mask, and a method for producing a semiconductor device, the exposure mask being a reflective type mask being capable of dealing with extreme ultraviolet light, the film thickness of an absorber film being decided so that the line width variation and pattern shift of a pattern exposed on a wafer are at their minimums and so that the exposure mask contributes to the improvement of the performance of a semiconductor device (appropriate address for miniaturization).